1. Field of the Invention
This invention relates to an apparatus and process for dehydrating waste solids concentrates, especially sewage sludge. More particularly, it relates to an apparatus and process wherein such concentrates, kept pumpable by having been mixed with fluidizing oil, are subjected to repeated dehydrations at successively higher concentrations.
2. Discussion of the Prior Art
The economic disposal of sewage sludge is a recognized problem. Processes and apparatus for dehydrating sewage sludges and other waste solids concentrates have been developed to deal with problems of disposal, avoidance of pollution, hygienic handling, economic operation, and the like. In those processes by-products are produced which are either valuable in themselves or can be utilized to further the process in some way.
In treating sewage it is customary to settle the raw sewage, generating a primary sewage sludge and a separated water fraction. That fraction is aerated to provide a secondary sewage sludge, usually but not necessarily an activated sludge, and clean water of quality suitable for discharge into a stream or river. Both primary and secondary sludges are discharged into a digester which is a closed tank in which the sludge may have a dwell time of up to 30 days or longer. There, heating coils may maintain the sludge at a somewhat elevated temperature to accelerate anaerobic bacterial digestion of organic constituents and the breakdown of solids, in part, into gaseous materials. The gaseous materials have a sufficient methane content to be useful as fuel for heating water circulated in the digester coils. The digested material is withdrawn as a stream of sewage solids or digested sludge which may have a water content of about 95%, and is spread out in large drying beds for drying by evaporation. The resulting dry solids may be burned, either supplying heat to the process or in uses as fuel outside the process, or they may be employed otherwise, for example, as fertilizer.
This invention is concerned with an improvement in an already improved apparatus and process to be used instead of the digester, or at least instead of the drying beds customarily associated therewith. The basic, known equipment needed to carry out the process operations is far more compact and appreciably less expensive than digester equipment and drying beds, particularly where the drying beds must be enclosed to control odors and permit year-round use.
The apparatus and process of which the present invention provides an improvement related to drying an initially water-containing waste solids concentrate, particularly sewage sludge, by admixing it with a fluidizing oil to obtain a mixture which remains fluid and pumpable even when the water content thereof has been removed. The resulting oil-containing mixture is passed through a sequence of dehydration steps in which the mixture is dehydrated by heat evaporation, becoming increasingly dehydrated. In a generally preferred or at least fairly widely practiced embodiment of the known art, economies of energy consumption are realized by utilizing the evolved vapors from each evaporation step in a "backward flow" arrangement to supply a substantial portion of the heat requirements of the preceding evaporation step, with the dehydration occurring at progressively higher temperatures as described in U.S. Pat. No. Re. 26,317. The process of that patent generates a substantially anhydrous slurry of sewage solids in oil which is withdrawn and further treated in a centrifuge to provide a substantially oil-free as well as dry sewage solids product and recover at least most of the fluidizing oil for re-use in the process. Further employments of the oil-fluidizing of initially water-containing waste solids concentrates for dehydration in single-stage evaporators and multiple-stage, "forward flow" evaporators are disclosed respectively in U.S. Pat. No. Re. 26,352 and U.S. Pat. No. 3,323,575.
By way of definition, with regard to evaporators in which fluid mixtures may be dehydrated in a plurality of sequential steps as well as those in which all dehydration takes place in a single stage, the expression "first stage" refers to that part of the evaporator equipment in which the hydrated mixture or wet slurry is subjected to the first step of the aforesaid sequential plurality of dehydration steps, two or three or more corresponding to "first stage", "second stage", "third stage", etc. The expression "effect", on the other hand, as in "multiple-effect", "first effect", "second effect", etc., is related to the flow and action of the heating material, steam here, in the evaporator equipment. Where the flow of the wet slurry being progressively heated and dried is countercurrent to that of the heating steam from stage to stage or effect to effect, the operation particularly illustrated and described in U.S. Pat. No. Re. 26,317 aforesaid and in the present case representing the condition of "backward flow", the first stage of a multi-effect or multiple-effect evaporator is the same as its last effect. Conversely, where the flow of wet slurry is cocurrent with that of the heating steam from stage to stage or effect to effect, the operation particularly illustrated and described in U.S. Pat. No. 3,323,575 aforesaid representing the condition of "forward flow", the first stage and first effect are the same as indeed they are in a single-stage or single-effect evaporator according to the disclosures of U.S. Pat. No. Re. 26,352 aforesaid. It is not especially meaningful to describe a single-stage or single-effect evaporator as operating in either backward flow or forward flow.
Sewage sludge, hereinafter sometimes simply called sludge, treated by the cited processes generally comprises non-fat solids in an amount of 2 to 50 or more weight percent, usually 3 to 35 weight percent; fluid fats or oils on a dry basis in an amount of about 1.0 to 30 weight percent, usually 8 to 20 weight percent, with the remainder of the mixture being predominantly water. The solids particle size is normally less than about one-quarter inch, encompassing the normal distribution in sewage sludge. Larger particles, such as occur in garbage, should be comminuted prior to being treated according to the dehydration process of the invention.
Oils useful in those processes for admixture with sewage sludge to provide the necessary fluidization are inert, relatively non-volatile oils or fats, or other non-volatile oil-like materials or the non-volatile fractions thereof. Typical of those oils are tallow, animal fats and vegetable oils, all of which can be derived directly from the process, depending upon the particular material or materials treated and also petroleum oils, fuel oils, glycerides, glycols and mixtures thereof, including industrial by-products. The quantity of oil used is such that its ratio in the system is approximately 2 to 20 times the weight of the non-fat solids. That refers to total oil, i.e., that added plus that derived from the process for reuse. Such an amount of oil is sufficient, under normal conditions, to give or maintain the condition of a fluid, pumpable mixture even when the water content is or has been removed from the mixture.
It has been noted recently that, in the course of concentrating certain sludges in a multi-effect evaporator system of the type described in Pat. No. Re. 26,317 and here, using a fluidizing oil carrier, there can be an increase in the viscosity of the oil slurry in some stage which reduces the heat transfer rate and results in a reduction in concentration capacity. It has been found that in sludges which, prior to fluidizing, have solid contents in the range of 10% to 35% on an oil-free basis, and which contain appreciable amounts of secondary sludge or digested sludge, there is an increase in viscosity during the concentration process which is sometimes accompanied by fouling of the heat transfer surfaces. In addition such sludges are difficult to fluidize, even with the addition of large quantities of fluidizing oil, often resulting in a difficult sludge pumping problem between the fluidizing tank and the evaporator system or array. That appears to be due to insufficient wetting of the sludge particles by the fluidizing oil which results in agglomeration of the particles, the formation of solid clumps in a separate phase from the added oil, and consequent reduction in fluidity of the sludge-oil mixture.
While dispersing agents can be added to the slurry system, oil-soluble agents are usually required and their use results in added operating costs because of the difficulty of recovering them. Attempts to improve the mixing operation by intensifying the mixing cause formation of an oil-sludge emulsion which results in further thickening. The oil-sludge emulsion can sometimes coat the heat transfer surfaces of the evaporator, fouling and reducing the heat transfer coefficient of those surfaces. Such a condition can lead to plugging of evaporator tubes. Those problems have been found to be particularly difficult when a petroleum oil, rather than an animal or vegetable oil or fatty acid, is used as the fluidizing oil medium or carrier.
It would be desirable, therefore, to avoid a physical change in an oil-sludge or sludge or sewage solids mixture being dehydrated in an evaporator as described above, which change manifests itself as and results in the presence of a coalesced clump or mass or emulsion adversely affecting the viscosity of the system and reducing the heat transfer efficiency or capability of the evaporator.